Denny, I don't think any of the oils will kill a transmission. However, rears need the EP additives for the hypoid gears. I used straight mineral 90w in transmissions for many years, because that was all there was and I changed it annually. When synthetic came out, I switched it and I never change it. I have never had a transmission failure with synthetic 50 and I have run some of them a million miles without changing oil. This is a lot farther then the recommended intervals, but the transmissions are out of warranty is out anyway. My opinion is that if you are going to put out the money for synthetic, why not put the lubricant in that was designed for transmissions. I have not changed fluids in a transmission in or rear in 20 years. The only failures I have had was the power divider on rear ends and that was due to the driver leaving the power divider locked on hard ground. I have been thinking of mounting something like a backup alarm inside the cab to remind them the power divider is locked.
